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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 2. Overview</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="Linux-PAM_ADG.html" title="The Linux-PAM Application Developers' Guide"><link rel="up" href="Linux-PAM_ADG.html" title="The Linux-PAM Application Developers' Guide"><link rel="prev" href="adg-introduction-synopsis.html" title="1.2. Synopsis"><link rel="next" href="adg-interface.html" title="Chapter 3.  The public interface to Linux-PAM"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 2. Overview</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="adg-introduction-synopsis.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="adg-interface.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="adg-overview"></a>Chapter 2. Overview</h1></div></div></div><p>
      Most service-giving applications are restricted. In other words,
      their service is not available to all and every prospective client.
      Instead, the applying client must jump through a number of hoops to
      convince the serving application that they are authorized to obtain
      service.
    </p><p>
      The process of <span class="emphasis"><em>authenticating</em></span> a client is what
      PAM is designed to manage. In addition to authentication, PAM provides
      account management, credential management, session management and
      authentication-token (password changing) management services.  It is
      important to realize when writing a PAM based application that these
      services are provided in a manner that is
      <span class="emphasis"><em>transparent</em></span> to the application. That is
      to say, when the application is written, no assumptions can be made
      about <span class="emphasis"><em>how</em></span> the client will be authenticated.
    </p><p>
      The process of authentication is performed by the PAM library via a
      call to <code class="function">pam_authenticate()</code>. The return value
      of this function will indicate whether a named client (the
      <span class="emphasis"><em>user</em></span>) has been authenticated. If the PAM library
      needs to prompt the user for any information, such as their
      <span class="emphasis"><em>name</em></span> or a <span class="emphasis"><em>password</em></span>
      then it will do so. If the PAM library is configured to authenticate
      the user using some silent protocol, it will do this too. (This
      latter case might be via some hardware interface for example.)
    </p><p>
      It is important to note that the application must leave all decisions
      about when to prompt the user at the discretion of the PAM library.
    </p><p>
      The PAM library, however, must work equally well for different styles
      of application. Some applications, like the familiar
      <span class="command"><strong>login</strong></span> and <span class="command"><strong>passwd</strong></span> are terminal
      based applications, exchanges of information with the client in
      these cases is as plain text messages. Graphically based applications,
      however, have a more sophisticated interface. They generally interact
      with the user via specially constructed dialogue boxes. Additionally,
      network based services require that text messages exchanged with the
      client are specially formatted for automated processing: one such
      example is <span class="command"><strong>ftpd</strong></span> which prefixes each exchanged
      message with a numeric identifier.
    </p><p>
      The presentation of simple requests to a client is thus something very
      dependent on the protocol that the serving application will use. In
      spite of the fact that PAM demands that it drives the whole
      authentication process, it is not possible to leave such protocol
      subtleties up to the PAM library.  To overcome this potential problem,
      the application provides the PAM library with a
      <span class="emphasis"><em>conversation</em></span> function.  This function is called
      from <span class="emphasis"><em>within</em></span> the PAM library and enables the PAM
      to directly interact with the client. The sorts of things that this
      conversation function must be able to do are prompt the user with
      text and/or obtain textual input from the user for processing by the
      PAM library. The details of this function are provided in a later
      section.
    </p><p>
      For example, the conversation function may be called by the PAM
      library with a request to prompt the user for a password. Its job is
      to reformat the prompt request into a form that the client will
      understand. In the case of <span class="command"><strong>ftpd</strong></span>, this might involve
      prefixing the string with the number <span class="command"><strong>331</strong></span> and sending
      the request over the network to a connected client. The conversation
      function will then obtain any reply and, after extracting the typed
      password, will return this string of text to the PAM library. Similar
      concerns need to be addressed in the case of an X-based graphical
      server.
    </p><p>
      There are a number of issues that need to be addressed when one is
      porting an existing application to become PAM compliant. A section
      below has been devoted to this: Porting legacy applications.
    </p><p>
      Besides authentication, PAM provides other forms of management.
      Session management is provided with calls to
      <code class="function">pam_open_session()</code> and
      <code class="function">pam_close_session()</code>. What these functions
      actually do is up to the local administrator. But typically, they
      could be used to log entry and exit from the system or for mounting
      and unmounting the user's home directory. If an application provides
      continuous service for a period of time, it should probably call
      these functions, first open after the user is authenticated and then
      close when the service is terminated.
    </p><p>
      Account management is another area that an application developer
      should include with a call to <code class="function">pam_acct_mgmt()</code>.
      This call will perform checks on the good health of the user's account
      (has it expired etc.). One of the things this function may check is
      whether the user's authentication token has expired - in such a case the
      application may choose to attempt to update it with a call to
      <code class="function">pam_chauthtok()</code>, although some applications
      are not suited to this task (<span class="command"><strong>ftp</strong></span> for example)
      and in this case the application should deny access to the user.
    </p><p>
      PAM is also capable of setting and deleting the user's credentials with
      the call <code class="function">pam_setcred()</code>. This function should
      always be called after the user is authenticated and before service
      is offered to the user. By convention, this should be the last call
      to the PAM library before the PAM session is opened. What exactly a
      credential is, is not well defined. However, some examples are given
      in the glossary below.
    </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="adg-introduction-synopsis.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="adg-interface.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">1.2. Synopsis </td><td width="20%" align="center"><a accesskey="h" href="Linux-PAM_ADG.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 3. 
      The public interface to <span class="emphasis"><em>Linux-PAM</em></span>
    </td></tr></table></div></body></html>
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